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> Forsiden > Publikationer > Geology of Greenland Survey Bulletin > Vol. 191 Geol. Greenl. Surv. Bull. > Review of Greenland Activities 2001, pp 24-32

GEOLOGY OF GREENLAND SURVEY BULLETIN 191

 
The Precambrian supracrustal rocks in the Naternaq (Lersletten) and Ikamiut areas, central West Greenland

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Naternaq, or Lersletten, in central West Greenland is an
extensive Quaternary outwash plain characterised by
light grey, silty sediments. Scattered low hills with out-
crops of crystalline Precambrian basement rocks pro-
trude from the outwash plain and form the northern part
of the Nagssugtoqidian orogen (e.g. Connelly et al.
2000). The prominent Naternaq supracrustal belt, at
least 25 km long and up to c. 2 km wide, occurs along
the north-western margin of Lersletten, bordered on
both sides by Archaean orthogneisses and granitic rocks;
the supracrustal rocks outline a major fold structure
with an irregular and sporadically exposed hinge zone
(Fig. 1). The supracrustal rocks, including the fold clo-
sure, exhibit a negative signature on the regional aero-
magnetic map (Fig. 2). The belt is known for its
disseminated and massive iron sulphide mineralisation
with minor copper and zinc, which is common in the
south-eastern part of the belt.
Study of the Naternaq supracrustal belt was an impor-
tant objective of the Survey's field work in 2001 (Stendal
et al. 2002, this volume). Boundaries, contact relationships
and principal rock types were established in the western
part of the belt (van Gool et al. 2002, this volume). Well-
exposed parts of the belt were mapped in detail (Fig. 3),
and the main lithologies and their mineralisations were
investigated and sampled. Preliminary results of the map-
ping are presented in this report, together with a brief
discussion of the depositional environment, likely age, and
mineralisation processes of the supracrustal sequence.
Large tracts of supracrustal rocks in the adjacent Ikamiut
area (Fig. 1) and near Qasigiannguit/Christianshåb some
50 km further to the east may once have been contigu-
ous with the Naternaq supracrustal belt, but further map-
ping is required to substantiate this.
Previous work
The earliest coastal geological reconnaissance in cen-
tral West Greenland was carried out by Noe-Nygaard
& Ramberg (1961), who noted the obvious supracrustal
origin of garnet-mica schists at Ikamiut and in the
Qasigiannguit area. The Naternaq supracrustal belt itself
was outlined by Henderson (1969) on his preliminary
map of the Egedesminde­Christianshåb area.
Kryolitselskabet Øresund A/S (KØ) undertook a major
base metal exploration programme in the region in
1962­1964, which concentrated on the most extensively
mineralised southern part of the Naternaq supracrustal
belt, following the discovery of the mineralisation in 1962
(Keto 1962; Vaasjoki 1965). General geological, elec-
tromagnetic, magnetic and gravimetric ground surveys
were carried out which helped define drilling targets
characterised by anomalous electromagnetic signatures
and Cu-Zn geochemical anomalies. These were drilled
and trenched, and indicate a sulphide resource of 2.4­4.8
million tonnes grading 30­35 wt% Fe and locally up to
2.7% Cu and 3.75% Zn (Vaasjoki 1964, 1965). In 1978
a regional airborne magnetic and electromagnetic sur-
vey by KØ covered parts of central West Greenland
including Naternaq (Peltonen 1978). In 1990­1993
Nunaoil A/S (now Nuna Minerals A/S) prospected at
Naternaq with ground geophysical VLF and magnetic
surveys, as well as a regional sediment sampling pro-
gramme (Gowen 1992; Sieborg 1992; Grahl-Madsen
1994). Nunaoil also re-analysed the KØ drill cores for gold,
and found Au values of up to 0.6 ppm over 0.35 m.
These exploration programmes only paid limited atten-
tion to the genesis and structural evolution of the host
rocks and their mineralisation processes.
In 1992 a high resolution aeromagnetic survey was car-
ried out in central West Greenland by Geoterrex Ltd
(Canada), financed by Danish and Greenlandic sources
(Thorning 1993). Part of the resulting aeromagnetic map
is shown in Fig. 2. A regional interpretation of the aero-
magnetic data by Schacht (1992) distinguished several
areas of supracrustal rocks, including those at Naternaq,
as well as other geological features. The Lersletten
supracrustal belt stands out as a prominent magnetic
low, whereas orthogneisses generally produce banded
24
The Precambrian supracrustal rocks in the Naternaq
(Lersletten) and Ikamiut areas, central West Greenland
Claus Østergaard, Adam A. Garde, Jeppe Nygaard, Jette Blomsterberg, Bo Møller Nielsen,
Henrik Stendal and Christopher W.Thomas
Geology of Greenland Survey Bulletin 191, 24­32 (2002) © GEUS, 2002
GSB191-Indhold 13/12/02 11:29 Side 24
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25
68
°
30
68
°
30
52
°
Naajannguit
10 km
Naternaq/
Lersletten
Greenland
Quaternary deposits
Undifferentiated amphibolite
Mica schist and other
metasedimentary rocks
Tonalitic­granodioritic­
granitic orthogneiss
Sydostbugten
Puagiarsuup Ilua
Nivaap Paa
Ikamiut
Fig. 3
Fig. 1. Geological sketch map of the
Naternaq­Ikamiut area based on field
work in 2001 and reconnaissance data
from Henderson (1969). The inset map
shows the position in West Greenland
(arrow), and the location of Fig. 3 is
shown by a red frame.
262
221
185
164
150
137
127
118
111
104
98
92
87
81
76
70
66
62
58
54
50
46
41
36
31
25
20
14
8
0
­10
­21
­32
­43
­57
­73
­89
­106
nT
68
°
30
68
°
30
52
°
52
°
10 km
Fig. 2. Magnetic total-field
intensity map with shaded
relief of the Naternaq­
Ikamiut area, the same area
as shown on Fig. 1. The
magnetic patterns clearly
reflect the major fold
structure and lithologies
evident from Fig. 1. The
shading was undertaken
with an inclination of 20°
and illumination from 330°.
Data from the Aeromag
1992 programme (Schacht
1992; Thorning 1993).
GSB191-Indhold 13/12/02 11:29 Side 25
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linear anomalies of intermediate amplitude. Amphibolite,
with intermediate to very high amplitudes, can be diffi-
cult to distinguish from orthogneiss. They are both vis-
ible as clusters of high amplitude anomalies with
half-widths in the order of 1­2 km. Mica schists produce
banded linear zones with low magnetic response, whereas
granites produce equidimensional, homogeneous zones
with high response.
Sparse geochronological data from the Naternaq area
suggest that the Naternaq supracrustal belt itself is of
Palaeoproterozoic age, whereas the supracrustal rocks
at Ikamiut are probably Archaean. Kalsbeek (1993) and
Kalsbeek & Taylor (1999) obtained a Palaeoproterozoic
Rb-Sr whole-rock age from metasedimentary rocks at
Ikamiut which shows that this area was reworked dur-
ing the Nagssugtoqidian orogeny, but the inferred ini-
tial
87
Sr/
86
Sr ratio is so high that these rocks were
interpreted as Archaean. Ion probe U-Pb age determi-
nations carried out by the Danish Lithosphere Centre
in the mid-1990s of a few zircons from one of the gra-
nodioritic to granitic orthogneiss bodies that appear to
cut the supracrustal belt indicated an emplacement age
between 2750 and 2900 Ma (Kalsbeek & Nutman 1996);
however, details of the contact relationships between
the orthogneisses and supracrustal rocks were not
reported, and the contact may be tectonic. Four zir-
cons were also analysed from a rock interpreted in the
field as a meta-andesite, which was collected from the
apparent continuation of the Naternaq supracrustal belt
10 km to the east of the area shown in Fig. 1. One of
the zircons gave an Archaean age, whereas the three
others gave ages of 1900­2000 Ma that could either be
metamorphic or protolith ages. Preliminary
207
Pb/
206
Pb
ages of around 2000 Ma recently obtained from detri-
tal or volcanic zircons in a mica schist near `Finger Lake'
(Fig. 3; J.N. Connelly & K. Thrane, personal communi-
cation 2002) support the latter interpretation.
Naternaq supracrustal belt
The Naternaq supracrustal belt outlines a major, com-
posite, overturned fold with an amplitude of about 25 km
and straight limbs (Fig. 1). The thicker, south-eastern limb
is up to c. 2 km wide, subvertical, and seems to extend
c. 60 km towards the east-north-east; a large part is con-
26
Unexposed (inside the area of supracrustal rocks)
Tonalitic­granodioritic­granitic orthogneiss
Pale colours: inferred lithologies
Amphibolite
Archaean
Marble, calc-silicate rocks and local quartzite
Exhalitic rocks, locally with sulphide mineralisation
Banded iron-formation
BIF
Siliceous muscovite schist and garnet-mica schist
Palaeoproterozoic
`Round Lake'
`Finger Lake'
Granite and pegmatite
0
3 km
Figs 6, 7
Fig. 5
Fig. 4
BIF
BIF
52
°
52
°
68
°
25
68
°
25
Fig. 3. Detailed geological map from the southern part of the Naternaq supracrustal belt. `Round Lake' and `Finger Lake' are informal
KØ names (see the main text). The position in the Naternaq area is shown on Fig. 1. Positions of photographs (Figs 4­7) are shown
by arrows.
GSB191-Indhold 13/12/02 11:29 Side 26
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cealed by the Lersletten Quaternary deposits. The some-
what thinner north-western limb is c. 25 km long and has
steep south-easterly dips. Axes of small-scale folds in the
hinge zone, interpreted as representative for the orien-
tation of the major fold, generally plunge moderately to
steeply to the south-east. Steeply west-plunging folds are
found within the south-eastern limb, which is commonly
intensely folded internally. Outcrop patterns in different
parts of the poorly exposed hinge zone suggest that the
major fold structure is the result of more than one phase
of folding, and refolded folds have been observed at a
number of localities. It has, however, not been possible
to establish a detailed deformation history. Intensely
deformed horizons of supracrustal rocks with a northward
concave flexure immediately to the west of the major
fold may indicate the presence of a late NNE­SSW-trend-
ing (sinistral?) shear zone in this area.
Contacts between the supracrustal rocks and the
regional tonalitic­granodioritic orthogneiss along the
limbs of the major fold are generally sharp and intensely
deformed, but the primary age relationships and nature
of the contact are ambiguous. Variably deformed to
almost undeformed masses of pink granite-pegmatite
are particularly common in the hinge zone and post-
date the orthogneiss. They are locally clearly discordant
to the intensely deformed marginal part of the
supracrustal belt and may have been emplaced during
the development of the major overturned fold.
Three main rock types dominate the Naternaq
supracrustal belt: amphibolite, fine-grained siliceous
quartzo-feldspathic rocks, and garnet-mica schist.
Quartzitic rocks, marble and calc-silicate rocks, car-
bonate and oxide facies iron-formation, and chert-rich
layers interpreted as exhalites, are minor constituents.
In spite of internal folding and possibly thrusting the
supracrustal belt possesses a crude lithological stratig-
raphy which is interpreted as a primary feature.
Amphibolite is consistently found along the outer mar-
gin of the major fold, succeeded towards the core by
fine-grained siliceous quartzo-feldspathic rocks alter-
nating with garnet-mica schist in which other horizons
of amphibolite are intercalated. Where marble is pre-
sent, mostly intercalated with calc-silicate rocks, it imme-
diately succeeds the marginal amphibolite (Fig. 3).
Amphibolitic rocks
The amphibolites in most parts of the supracrustal belt
are granoblastic to intensely schistose or lineated, mostly
very fine-grained rocks primarily composed of horn-
blende and plagioclase, commonly with minor quartz
and biotite, and locally garnet. Varieties with deformed,
1­5 cm long plagioclase aggregates (possibly former
phenocrysts) have also been observed. In places
medium-grained, homogeneous, plagioclase-rich `grey
amphibolite' and hornblende-quartz rocks occur. The
main c. 200­400 m thick amphibolite unit at the mar-
gin of the belt commonly contains irregular, intercon-
nected calc-silicate bands and lenses which are up to a
few centimetres thick and more or less conformable
with the main foliation; the adjacent amphibolite is com-
monly garnet-bearing. Locally, the calc-silicate rich layers
are up to c. 20 cm thick and may be followed for tens
of metres along strike. The fine-grained, calc-silicate
banded amphibolites are interpreted as variably spilitised
or hydrothermally altered, and subsequently strongly
deformed, lavas and breccias. Small bodies of medium-
grained, foliated to granoblastic, hornblende- or plagio-
clase-porphyroblastic varieties of probable intrusive
origin also occur. Up to 10 m thick layers of biotite
(-garnet) schist are common within the amphibolite. In
the westernmost part of the hinge zone, as well as in
other parts of the marginal amphibolite, 1­30 cm thick
layers of very fine-grained siliceous, muscovite- and
biotite-bearing, pale grey metasedimentary or meta-
volcanic rocks are interleaved with the amphibolite.
Chemical sedimentary rocks, banded iron-
formation and sulphide mineralisation
In the southern and north-western parts of the
supracrustal belt, especially near the lakes designated
`Round Lake' and `Finger Lake' by KØ (Fig. 3), the mar-
ginal amphibolite is succeeded by an irregular, and in
most places intensely deformed sequence of chemical
sediments. These mainly consist of marble, with minor
carbonate and oxide facies banded iron-formation and
cherty exhalites, the latter locally with semi-massive to
massive sulphide mineralisation (see below). The
detailed map of this area (Fig. 3) shows up to three sep-
arate sequences of marble and exhalites with sulphide
mineralisation, which is due to repetition of a single orig-
inal sequence by folding.
The scattered, up to c. 100 m thick marble occurrences
near `Round Lake' and `Finger Lake' (Fig. 4) mainly
consist of impure, greyish to brownish weathering, fine-
grained dolomitic marble, commonly with centimetre-
to decimetre-thick intercalations of calcite marble and
calc-silicate rocks dominated by tremolite-actinolite +
diopside ± dolomite and late talc. The lack of forsterite
and the presence of sillimanite in adjacent pelitic rocks
suggest P­T conditions of approximately 650 ± 50°C at
27
GSB191-Indhold 13/12/02 11:29 Side 27
background image
4.5 ± 0.5 kbar; tremolite-actinolite may have grown dur-
ing decreasing temperature. A few other marble hori-
zons up to about two metres thick have also been found
at approximately the same stratigraphic level, e.g. in the
hinge zone of the major fold. The banded iron-forma-
tion forms sporadic, 0.5­1 m thick layers on the south-
eastern fold flank along strike of the dolomite marble,
and consists of alternating centimetre-thick layers of
dolomite, magnetite, siderite, quartz and calc-silicate
minerals. A single, larger, c. 30
x 40 m fold closure with
the same type of banded iron-formation crops out east-
north-east of `Finger Lake' (Fig. 5).
A range of variably altered, conformable horizons of
very fine-grained siliceous and sulphidic lithologies
associated with either amphibolite or marble are inter-
preted as volcanogenic-exhalitic rocks. Light grey, finely
laminated (millimetre-scale) cherty rocks predominate
and usually contain up to c. 20% dark, very fine-grained
sulphidic seams. The fine lamination, which may be a
primary feature, is in most places destroyed by intense
secondary alteration characterised in the field by a sul-
phide-yellow, variably rusty appearance. Seams of fine-
grained dolomite and micaceous metasediments are
commonly intercalated with the mineralised cherty lay-
ers, resulting in composite, rusty weathering outcrops
with variable mineralogy.
The largest semi-massive to massive pyrrhotite-rich
sulphide mineralised zones are found near `Round Lake'
and `Finger Lake' (Fig. 3). Massive sulphides form up to
c. 2 m wide and 10 m long lenses (maximum size 2
x
10 m); the mineralised rocks are iron-rich and dominated
by pyrrhotite, with minor chalcopyrite and sphalerite (up
to c. 3%) and subordinate pyrite, arsenopyrite, magnetite
28
Fig. 4. Dolomitic marble in the southern
part of the Naternaq supracrustal belt
(location shown on Fig. 3).
Fig. 5. Carbonate-oxide facies banded iron-formation in the south-
ern part of the Naternaq supracrustal belt (location shown on Fig.
3). Hammer: 50 cm.
Fig. 6. Weathered-out chert-sulphide concretions of possible dia-
genetic origin in sulphide-mineralised host in the southern part
of the Naternaq supracrustal belt (location shown on Fig. 3).
Hammer: 50 cm.
GSB191-Indhold 13/12/02 11:29 Side 28
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and graphite. Thinner, conformable horizons containing
disseminated to semi-massive sulphides may be followed
for up to a few hundred metres. Within one of the min-
eralised zones at `Finger Lake', a number of 10­40 cm
chert-sulphide concretions were observed in a pyrrhotite-
rich, semi-massive sulphidic host rock (Fig. 6). It is con-
sidered likely that these concretions were formed during
the mineralising event itself, or the subsequent diagen-
esis. Disseminated sulphides are common in the host
amphibolite, marble and mica schist adjacent to the mas-
sive and semi-massive sulphide occurrences.
Metre-sized and larger, tight, overturned, angular
folds are common in the mineralised areas. Many of the
massive sulphide lenses occur in the hinge zones of such
small-scale folds (Fig. 7), indicating a certain degree of
hydrothermal sulphide remobilisation during the per-
vasive ductile deformation. However, detailed mapping
south-east of both `Round Lake' and `Finger Lake' shows
that these lenses could be stratigraphically connected
with each other. The sulphidic, exhalite horizons are
commonly extensively crushed along narrow, secondary
fault zones.
Quartzitic rocks
Discrete, 1­5 m thick, fine-grained quartzitic horizons s.s.
with thin magnetite-rich seams, presumably of clastic
sedimentary origin, are found locally adjacent to the
marble. Another quartzitic unit without magnetite seams,
but containing dispersed, fine-grained biotite, occurs on
the south-eastern limb of the major fold some 15 km
east of the area shown in Fig. 3. It has a strike length
of c. 2 km and is up to 30­40 m thick.
Fine-grained siliceous and pelitic
metasediments
The interior part of the supracrustal belt at Naternaq with
respect to the major overturned fold is a c. 200­300 m
thick succession of mainly siliceous, muscovite schists
together with minor biotite-garnet schists and amphi-
bolite. The siliceous schists are generally light grey to
light brown in colour and very fine-grained; they com-
monly have a very massive appearance without much
apparent lithological variation, and no primary struc-
tures have been observed. In some places they contain
up to centimetre-sized porphyroblasts or pseudomorphs
of andalusite. Garnet-rich horizons are common adja-
cent to amphibolite contacts. The origin of the siliceous
schists is not clear from their field appearance alone;
they may be metasedimentary or metavolcanic rocks,
or a mixture of both (see discussion).
Garnet-mica schists, commonly sillimanite-bearing,
generally fine- to medium-grained and with a strong
penetrative S fabric, are intercalated with the siliceous
schist and fine-grained amphibolite in layers from a
few centimetres to tens of metres thick. In some areas
the schists contain irregular to strongly planar quartzo-
feldspathic melt veins on a centimetre-scale, which give
them a migmatitic appearance.
Supracrustal rocks of the Ikamiut district
Supracrustal rocks crop out extensively south and west
of Ikamiut, the settlement between the bays of Nivaap
Paa and Sydostbugten in the south-east corner of Disko
29
Fig. 7. Sulphide mineralisation concen-
trated in the hinge areas of metre-scale
angular folds. Southern part of the
Naternaq supracrustal belt (location
shown on Fig. 3). Hammer: 50 cm.
GSB191-Indhold 13/12/02 11:29 Side 29
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Bugt (Fig. 1; Henderson 1969). The area was mapped
in some detail in August 2001, allowing refinement and
simplification of Henderson's original map and resolv-
ing some of the peculiarities of outcrop pattern that
arose from his reconnaissance mapping. The supracrustal
rocks are dominated by siliciclastic rocks, with subor-
dinate but important amphibolite horizons.
Relationships between orthogneisses and
supracrustal rocks
The supracrustal rocks are interlayered with granodi-
oritic to tonalitic orthogneisses. As far as could be deter-
mined, the interlayering is tectonic. Where contacts
between supracrustal rocks and orthogneisses can be
observed unambiguously, they are generally marked by
high strain and extensive mylonitic developments.
On the west coast of Naajannguit, 5.7 km due west
of Ikamiut in the northern part of the area (Fig. 1), a
unit of supracrustal rocks is sandwiched between ortho-
gneisses. The contacts on both sides of this supracrustal
unit are highly strained and marked by mylonites, indi-
cating tectonic interleaving. The orthogneisses seen
here seem to contain an additional phase of deforma-
tion to that affecting both the orthogneisses and the
supracrustal rocks as a whole.
Relationships between the orthogneisses and supra-
crustal rocks are somewhat obscure in the central part
of the Naajannguit area, where distinction between
siliceous and psammitic supracrustal lithologies and
orthogneisses is rendered difficult by similarities in com-
position and partial melting effects. No cross-cutting rela-
tionships that would indicate an intrusive relationship have
been observed, but such relationships may have been
obscured by the deformation. Such extremely limited
evidence as there is suggests that the supracrustal rocks
were deposited upon the orthogneisses, but this remains
to be confirmed. The orthogneisses and metasedimen-
tary rocks are disposed about kilometre-scale open to
tight upright folds that fold the penetrative fabric in the
rocks as well as the mylonitic contacts between the ortho-
gneisses and the metasedimentary rocks. A strong, pene-
trative stretching lineation is consistently parallel to the
WSW-plunge of the major folds.
Supracrustal lithologies
The dominant supracrustal lithologies are psammites,
micaceous psammites, schistose to gneissose pelites
and semipelites. Siliceous psammitic and quartzitic
lithologies are also present, and amphibolites form a
subordinate but important component. No unambigu-
ous evidence of younging was found, although possible
graded bedding was observed at one locality, suggest-
ing the rocks are the right way up.
The semipelitic and pelitic lithologies are well exposed
on the north-western side of Puagiarsuup Ilua. They are
coarse, schistose to gneissose migmatitic rocks with ubiq-
uitous thin, centimetre-scale, lenticular quartz-feldspar leu-
cosome veins, commonly with biotite-rich selvages.
Garnet is commonly abundant and is wrapped by the
penetrative schistose to gneissose fabric. Sillimanite is pre-
sent locally and, in one or two places, appears to be a
pseudomorph after kyanite. Sillimanite is also seen replac-
ing biotite. Locally, coarse quartz-plagioclase pegmatite
is abundant, forming metre-thick, lenticular bodies within
the micaceous host rock. Local garnet, muscovite and sil-
limanite in the pegmatites indicate derivation from a
metasedimentary source. Other `pegmatitic' bodies of
pale rock in this area are layered internally and may be
deformed psammites within the more pelitic rocks.
The psammitic rocks vary from medium- to coarse-
grained, dark, micaceous psammites to pale, quartzitic
rocks. Contacts with the semipelitic­pelitic lithologies
are transitional to sharp. The psammitic rocks are locally
garnet-bearing and, where amphibolites occur in adja-
cent outcrops, may also contain amphibole, suggesting
a volcanic input. The more micaceous psammitic rocks
are commonly migmatitic, with a quartz-feldspar leu-
cosome. While all the siliceous rocks are likely to be
sedimentary in origin, it is possible that some of them
may have been acid volcanic rocks.
Although subordinate in volume, there are significant
units of amphibolite, some of which contain abundant
large garnet porphyroblasts with very fine-grained quartz-
plagioclase pressure shadows. The amphibolites are
locally massive, but tend generally to be layered and
heterogeneous, with diopsidic and plagioclase-rich lay-
ers and marginal developments of thinly layered, calc-
silicate-bearing units. The amphibolite units are lenticular
and of limited lateral extent. A particularly fine sequence
of layered calc-silicate-bearing rocks crops out on the
peninsula north-west of Ikamiut, where they are char-
acterised by the presence of diopside with some dark
brown orthopyroxene. From their lithological character,
it is considered that the amphibolites and associated
rocks are most likely to be metavolcanic in origin. Sulphide
mineralisation occurs locally at the margins of amphi-
bolites, where calc-silicate-bearing units are developed.
30
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Regional correlation
The Ikamiut supracrustal rocks are, in general, very
similar to those described from Naternaq, although no
carbonate rocks, marbles, banded iron-formation or
layered cherts have been recorded in the Ikamiut area.
Thus the Ikamiut rocks are provisionally correlated with
those from Naternaq, with a possible continuation to
the north-east along strike from Ikamiut.
Discussion and conclusions
An interpretation of the depositional age and plate-tec-
tonic setting of the Naternaq supracrustal belt is essen-
tial for an evaluation of its economic potential. However,
the available geochronological data are not sufficient
to confidently determine its age. While observations
from adjacent areas in 2001 indicate that the orthogneiss
precursors are likely to have been intruded into
supracrustal packages (van Gool et al. 2002, this vol-
ume), in the Ikamiut area there are hints of a deposi-
tional unconformity.
Age determinations of orthogneisses farther south in
the Nagssugtoqidian orogen have shown that almost all
are Archaean. The only exception is the c. 1900 Ma
Arfersiorfik quartz diorite, which was emplaced between
the two Archaean continents that collided during the
Nagssugtoqidian orogeny and was involved in the ensu-
ing thrust stacking of Archaean and Palaeoproterozoic
crust in the central part of the orogen (e.g. Kalsbeek et
al. 1984, 1987; Connelly et al. 2000). Field observations
in 2001 by one of the authors (A.A.G.) suggest that the
northern limit of the thrust stack straddles 68°N. A pre-
liminary conjecture based on the structural and
geochronological data currently available indicates that
the orthogneisses in the Naternaq area belong to the
northern continent and are late Archaean in age, that
large fold structures farther to the south-west and hence
also the supracrustal rocks in that area are Archaean,
but that the Naternaq supracrustal belt and its fold struc-
tures are Palaeoproterozoic.
Due to ubiquitous high strain along the contacts
between the Naternaq supracrustal belt and the
orthogneisses it is not known with certainty whether
the former were deposited on the latter with a primary
depositional unconformity, or whether the two units are
tectonically interleaved. A third possibility, that the
gneiss precursors intruded the supracrustal sequence,
seems less likely in view of the recent age determina-
tions of zircons from the supracrustal belt reported
above. This unresolved problem has an important bear-
ing on the evaluation of the economic potential of the
supracrustal belt: is this a predominantly clastic epi-
continental sequence with a low economic potential,
or is it an arc-related, predominantly volcanic, bimodal
sequence comprising a lower, basic sequence (the mar-
ginal amphibolite) and an upper, acid sequence (the
siliceous mica schists), and thus with an interesting
base metal and gold potential? Further study of the
siliceous mica schists, including geochemistry and more
precise age determinations of their zircons, may pro-
vide an answer to this question. In addition, future
work may show that both Archaean and Palaeo-
proterozoic supracrustal rock sequences occur side by
side in the Naternaq area.
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32
Authors' addresses
C.Ø., Aggersvoldvej 15, 2. tv., DK-2700 Brønshøj, Denmark. E-mail: thin.air@get2net.dk
A.A.G., B.M.N. & H.S., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
J.N., Jægersborggade 2, 1. th., DK-2200 Copenhagen N, Denmark.
J.B., Bureau of Minerals and Petroleum, Government of Greenland, P.O. Box 930, DK-3900 Nuuk, Greenland.
C.W.T., British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK.
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